Further to my earlier response, I have made a quick search for clockmakers namned Söllner in Wien, from about 1850 onwards. The first I could find was Josef in 1931, but he was in Wien XIX (Döbling) which is a long way from Wien 3 (Landstrasse). He was not listed after 1934.
Then came Karl Söllner in 1941 and he was in Reisnerstrasse 9, right in the middle of Wien 3 (Landstrasse), so in just the right place. During the war many records were destroyed so am not sure what he was doing after 1941. However, a very recent record shows Maria Söllner, 'jewelery and clocks', still at the same address, Reisnerstrasse 9. She must surely be a relative.
The stamp on your clock is not, in my opinion, contemporary to the original date of manufacture. It would, however, be perfectly in keeping for a much later date, such as could have been put on by Karl or Maria Söllner. I don't know why, perhaps Söllner sold it at some point.
All this is of no help to know who actually made your clock, but many of these Viennese movements were never marked on movement or dial (and marks on the dial were often that of a retailer). But I hope it may throw a little light onto the origin of the mysterious stamp on the movement.